1. Field of the Invention
The present invention relates to a half bridge inverter and, more particularly, to a half bridge inverter controlled by a push/pull control chip to drive a load.
2. Description of Related Art
The power supply for a backlight source of a TFT LCD panel makes use of an inverter circuit to accomplish energy conversion and turn a cold cathode fluorescent lamp (CCFL) on. Conventional inverter circuits can be divided into half bridge-type, full-bridge type and push/pull-type according to different circuit topologies. An inverter circuit is a circuit for converting DC power into AC power.
As shown in FIG. 1, a transformer T1 divides the circuit into a front-end circuit at the first side 101 and a rear-end circuit at the second side 102. The front-end circuit at the first side 101 comprises a DC voltage source Vcc, a first switch Q1, and a second switch Q2. The rear-end circuit at the second side 102 comprises at least a capacitor (C1, C2, C3), a load, and at least a diode (D1, D2). A push/pull control chip 103 is connected between the front-end circuit at the first side 101 and the rear-end circuit at the second side 102.
Reference is also made to FIG. 2 as well as FIG. 1. The push/pull control chip 103 outputs a first control signal a and a second control signal b to turn switching actions of the two switches Q1 and Q2 at the first side 101, respectively. A DC power Vcc is used to provide energy, and the transformer T1 raises and converts the voltage of DC power Vcc to the rear-end circuit 102 for driving the load. The output voltage waveform c at the second side of the transformer T1 is the voltage waveform at point C. As shown in FIG. 2, the output voltage waveform c at the second side is an AC voltage waveform.
In the above description, the push/pull control chip 103 can be the LX1686, LX1688 or LX1691 push/pull control chip produced by Linfinity (Microsemi) Corporation, or the 02-9RR, 0Z9930, 0Z9938 or 0Z9939 push/pull control chip produced by O2 Micro International Limited, or the TL-494 or TL594 push/pull control chip produced by TEXAS INSTRUMENT, or the BIT3193, BIT3713, BIT3715 or BIT3501 push/pull control chip produced by Beyond Innovation Technology.
As shown in FIG. 3, a transformer T2 divides the circuit into a front-end circuit at the first side 201 and a rear-end circuit at the second side 202. The front-end circuit at the first side 201 comprises a DC voltage source Vcc, two electronic switches (Q1, Q2), a half-bridge control chip TL494, two capacitors (C1, C2) and a drive transformer Tr. The rear-end circuit at the second side 202 comprises a load.
Reference is also made to FIG. 4 as well as FIG. 3. The half-bridge control chip TL494 outputs control signals D1-D2 via two output terminals D1 and D2. The control signals D1-D2 control switching actions of the two electronic switches Q1 and Q2 via the drive transformer Tr, respectively. The two electronic switches Q1 and Q2 are N-MOSes or P-MOSes. Through switching actions of the two electronic switches Q1 and Q2, electric energy stored in the capacitors C1 and C2 can be transferred to a first side terminal T21 of the transformer T2 via a coupling capacitor C3 to form an AC power ac. The voltage of the capacitors C1 and C2 is a half (Vcc/2) of the DC voltage Vcc. The AC power ac is used to provide energy for the transformer T2, which boosts the AC power to the second side 202 for driving the load.
In the above description, if the inverter circuit used is of the half bridge-type, a half-bridge control chip needs to be matched for normal operations, while if the used inverter circuit is of the push/pull-type, a push/pull control chip needs to be matched for normal operations, hence having less flexibility and commonality in practical use. Because of the above limitation, control chips can't be jointly used and purchased together, or a more complicated circuit needs to be matched.
Moreover, the conventional half bridge inverter circuit needs to use an isolation transformer to control the switching actions of electronic switches instead of directly driving electronic switches. Besides, the two electronic switches used by the conventional half bridge inverter circuit are both N-MOSes, both P-MOSes, or a P-MOS and an N-MOS.